This invention relates to a deflection system overscan protection circuit for reducing excessive voltage and current generated by the deflection system.
In deflection systems, such as the vertical deflection system utilized in television receivers for providing scanning current for electromagnetic deflection coils, it is common to utilize oscillators synchronized by received sync pulses to provide the proper time base for the scanning current waveforms. The oscillators usually are of a free running type, and, with no sync pulse present, have their free running frequency adjusted to be somewhat less than the sync pulse frequency to enable reliable synchronization when the sync pulses are present.
To minimize circuit costs it is common practice to utilize circuit elements, such as resistors and capacitors, which have resistance and capacitance tolerances of 10 and 20 percent. Normally such utilization causes no problems because variable controls are utilized to set the frequency and amplitude of the deflection waveforms. However, to accomplish this the variable controls must have a relatively wide range of control to compensate for the tolerance variations of elements in a given circuit. Thus, for example, it is not uncommon for the free running frequency range of a vertical rate deflection oscillator having a normal frequency of 60 Hz to extend as low or lower than 40 Hz and as high or higher than 80 Hz.
At the relatively low vertical deflection rates, 50 or 60 Hz, utilized in most countries today the deflection coils present primarily a resistive load to the scanning current obtained from the deflection amplifier. The scanning current is generally a linear sawtooth waveform with some amount of nonlinearity added to provide "S-shaping" to ensure a linear raster as viewed on the picture tube viewing screen. Thus, the voltage and current of the scanning waveform increases substantially linearly with time. If for some reason the vertical oscillator frequency, or hold, control element is misadjusted, or there is some fault in the television receiver, or both, the vertical oscillator could operate in its free running condition at a much lower frequency, increasing the time between cycles and thereby increasing the scanning waveform voltage and current in the deflection amplifier. Because some of the deflection amplifier circuit elements such as capacitors may have a maximum voltage rating selected to be satisfactory under most operating conditions, such an overscan condition would result in the requirement of higher voltage ratings of the particular circuit element, increasing the receiver cost. Similarly, if the deflection waveform amplitude, or size, control were adjusted to an extreme providing a maximum current and voltage scanning waveform, particular circuit elements in the deflection amplifier could also be overstressed, requiring higher voltage rating devices. The worst condition could be when both the size control and the hold control were inadvertently set to result in scanning waveform increase in current and voltage. It is desirable to provide some inexpensive and automatic arrangement for reducing the effects of an overscan condition.